Packaging of all types of materials may require properties appertaining to functions involved in containing items and information about the use of such items. The container must have properties suited to the demands of the product. For instance, if the product is a liquid, then the container should be sealed to avoid spillage. If the liquid product is to be drunk, then the inside of the container must not contaminate the liquid nor cause its deterioration. Information of the product may be integral to the container or it may be separate. It may for instance have a function of aesthetically attracting a customer to the product or to instruct the customer how to use the product. As the product itself is generally what is being consumed, packaging provides an expense area that should perform its functions at a minimum cost. The printing of such packaging should have an optimum content of automation for this reason.
Modern technology has provided means of producing decorative and informative patterns of information on computers and it is desirable that this information be downloaded directly onto the packaging. One method of digital printing, which may be suitable for such applications, is inkjet. Inkjet is a non-impact printing process whereby ink is squirted through very fine nozzles and the resultant ink droplets form an image directly on a substrate. There are two main types of inkjet process. In one process, usually termed continuous inkjet printing (CIJ), a stream of ink drops are electrically charged and then deflected by an electric field either directly or indirectly onto the substrate. In the second process, usually called Drop on Demand (DOD) inkjet printing, the ink supply is regulated by an actuator such as a piezoelectric actuator. The pressure produced during the actuation forces a droplet through a nozzle onto the substrate. Inks for DOD inkjet printing do not need to be conductive.
WO 97/27053 by Jennel et al describes the use of inkjet to digitally write on packaging material. The printing can be done directly onto a pre-formed bottle such as one made from PET (polyester), or onto a carton blank or a web of packaging material. The invention is claimed to provide an advanced level of automation with minimum operator intervention. In order to achieve good adhesion to materials such as PET, ultra-violet (UV) sensitive inks are used and after jetting they are cured by UV radiation. The inkjet head is DOD and described as one supplied by the company Spectra. This is the most widely accepted way of using UV curing inkjet inks, as the alternative method, CIJ, generally uses water based inks and the inks must contain electrically conductive material. UV inks are generally based on organic acrylate mixtures that do not contain electrically conductive ingredients and are therefore less easily adapted for use in CIJ.
UV inkjet inks are more expensive than water-based inks and will remain so because by definition water-based inks contain a large quantity of water, which is relatively inexpensive. Because with UV inkjet inks all of the jetted material remains on the substrate surface (where the substrate is impermeable) inks are deposited in the form of tiny hemispherical structures. Process color work, where three or four separate inks are applied over the same area, can thus have a Braille-like feeling and such an effect limits print quality.
The use of water-based inks in packaging applications would be advantageous for several reasons. As has been mentioned above, they have cost advantages; they can be used in both DOD and CIJ inkjet systems and they do not pile-up because the major part of the inkjet drop is water, which disappears either by absorption, if the substrate is pervious, or by evaporation or both. However, there are a number of problems with using water-based inkjet inks in packaging. They have wetting problems with relatively low energy plastic surfaces (for instance that of PET) as well as slow drying, which for non-absorbent plastic surfaces has to occur only by evaporation. Also they have low wet smear resistance—i.e. after they have dried, they can be easily smeared with a wet finger. WO 99/21724 by Wang et als. addresses the problem of ink smearing. The patent application describes the use of two layers—an inner non-cross-linked hydrophilic coating and an outer cross-linked hydrophilic coating. In one embodiment, an inkjet image is applied before curing to avoid wet smear. Similarly, US 2001/0036552 by Otani et al. describes coating a substrate with two layers for water-based pigment inks to give better colors and image fastness.
It would be advantageous if a means could be found to produce a single coat for the substrate, which would be used to produce good quality water-fast images using aqueous inkjet inks.